1. Field of the Invention
The present invention relates to an intake air control apparatus for an engine with a rotational angle detection sensor for detecting the rotational angle of a throttle valve that adjusts the degree of opening in an intake passage.
2. Description of the Related Art
In a known intake air control apparatus for an conventional engine, there has been used a flux density detection type sensor provided with a Hall element as a sensor for detecting the rotational angle of a throttle valve fixedly secured to a shaft.
That is, a cylindrical measuring target, which has a magnetic circuit constructed by a permanent magnet and a magnetic member, is mounted on a sector-shaped final spur gear fixedly secured to a shaft which is driven to rotate by a drive motor, and a Hall element is arranged on the central axis of the measuring target, so that the Hall element can detect a change in the density of flux passing therethrough in accordance with the rotation of the measuring target operably connected with the final spur gear thereby to detect the rotational angle of the throttle valve (for example, see a first patent document: Japanese patent laid-open No. 2001-132494 (FIG. 1)).
In the intake air control apparatus for an engine as described above, the sensor for detecting the rotational angle of the throttle valve is arranged in the neighborhood of the drive motor that drives the shaft to rotate. With such an arrangement, there arises the following problem: that is, the sensor is subjected to the influence of the magnetic flux generated by the drive motor, and hence its output becomes unstable, so that the rotational angle of the throttle valve can not be accurately detected, resulting, for example, in a situation that a required amount of intake air can not be obtained.
To cope with such a situation, it is considered that the influence of magnetic flux from the drive motor is suppressed by enclosing the Hall element with a magnetic member. However, in the above-mentioned intake air control apparatus for an engine, the Hall element is arranged on the central axis of the measuring target, so the Hall element receives the magnetic flux of a permanent magnet from therearound. As a result, there arises another problem as stated below: that is, when the Hall element is enclosed with the magnetic member, it becomes impossible for the Hall element to accurately detect a change in the density of flux passing therethrough in accordance with the rotation of the measuring target.